Add ServingConfig model

This adds a `ServingConfig` model and a representation of the existing
default serving configuration, allowing us to remove the more specific
environment variable/app configuration.

app/models/serving_config.rb

@@ -0,0 +1,21 @@
+# Represents a serving config on Discovery Engine.
+#
+# A serving config is an endpoint on an engine that can be used for
+# querying. Each serving config can have different configuration (in particular, different sets of
+# active controls), which allows us to test out new configuration changes outside of the default
+# serving config.
+#
+# see https://cloud.google.com/ruby/docs/reference/google-cloud-discovery_engine-v1beta/latest/Google-Cloud-DiscoveryEngine-V1beta-ServingConfig
+ServingConfig = Data.define(:remote_resource_id) do
+  include DiscoveryEngineNameable
+
+  # The default serving config automatically available on an engine
+  def self.default
+    new("default_search")
+  end
+
+  def parent
+    # We only use a single engine in our architecture, so we can hardcode it here.
+    Engine.default
+  end
+end

app/services/discovery_engine/query/search.rb

@@ -56,7 +56,7 @@ def query
     end
 
     def serving_config
-      Rails.configuration.discovery_engine_serving_config
+      ServingConfig.default.name
     end
 
     def page_size

config/application.rb

@@ -18,7 +18,6 @@ class Application < Rails::Application
 
     # Google Discovery Engine configuration
     config.discovery_engine_default_collection_name = ENV.fetch("DISCOVERY_ENGINE_DEFAULT_COLLECTION_NAME")
-    config.discovery_engine_serving_config = ENV.fetch("DISCOVERY_ENGINE_SERVING_CONFIG")
     config.google_cloud_project_id = ENV.fetch("GOOGLE_CLOUD_PROJECT_ID")
 
     # Document sync configuration

spec/models/serving_config_spec.rb

@@ -0,0 +1,15 @@
+RSpec.describe ServingConfig do
+  subject(:serving_config) { described_class.new("my-serving-config") }
+
+  describe ".default" do
+    it "returns the default serving config" do
+      expect(described_class.default).to eq(described_class.new("default_search"))
+    end
+  end
+
+  describe "#name" do
+    it "returns the fully qualified name of the serving config" do
+      expect(subject.name).to eq("[collection]/engines/govuk/servingConfigs/my-serving-config")
+    end
+  end
+end

spec/services/discovery_engine/query/search_spec.rb

@@ -16,8 +16,6 @@
   end
 
   before do
-    allow(Rails.configuration).to receive(:discovery_engine_serving_config)
-      .and_return("serving-config-path")
     allow(DiscoveryEngine::Query::Filters).to receive(:new).and_return(filters)
   end
 
@@ -52,7 +50,7 @@
 
       it "calls the client with the expected parameters" do
         expect(client).to have_received(:search).with(
-          serving_config: "serving-config-path",
+          serving_config: ServingConfig.default.name,
           query: "garden centres",
           offset: 0,
           page_size: 10,
@@ -76,7 +74,7 @@
 
         it "calls the client with the expected parameters" do
           expect(client).to have_received(:search).with(
-            serving_config: "serving-config-path",
+            serving_config: ServingConfig.default.name,
             query: "garden centres",
             offset: 11,
             page_size: 22,